1. Field
The disclosure relates to envelope tracking for power amplifiers.
2. Background
Envelope tracking is a technique for increasing the efficiency of power amplifiers. In an envelope tracking (ET) system, the supply voltage of a power amplifier is dynamically adjusted to keep the power amplifier operating with sufficient headroom to maintain linearity, while nevertheless minimizing DC power consumption. The supply voltage of the power amplifier (or “tracking supply voltage”) may be generated using a separate linear amplifier that tracks the envelope of the power amplifier output. In certain implementations, the linear amplifier is itself coupled to an amplifier supply voltage generated by a boost converter, which is capable of generating a boosted supply voltage for the linear amplifier that exceeds the maximum supply voltage otherwise available to the system, e.g., from a battery of the system. In this manner, the power amplifier output can reach and even exceed the battery supply voltage when necessary. The tracking supply voltage may further be coupled to a switching power stage to provide greater driving capability to the power amplifier.
In certain cases wherein the battery supply voltage is low, and the power amplifier is called on to deliver a high peak power level, the tracking supply voltage may exceed the battery supply voltage. This may lead to a condition known as “reverse current flow,” wherein current flows from the tracking supply voltage through a high-side switch of the switching power stage back to the battery supply voltage. Reverse current flow undesirably degrades system efficiency, and requires the boost converter to be significantly over-designed. Furthermore, reverse current flow may introduce distortion into the power amplifier output, causing the output waveform to potentially violate linearity and RX band noise specifications.
It would be desirable to provide techniques for detecting and preventing reverse current flow in an envelope tracking system.